1. Field of the Invention
The invention relates to a field-effect transistor that uses superconductivity and, more particularly, to a field-effect transistor, the channel of which is made alternately superconductive or resistive by the application of an electrical field to the gate.
Components based on a superconductive effect have two main fields of application: these are the field of ultrasensitive detection (especially in the infrared) and that of logic applications with very little dissipation and high switching speeds. However, it does not appear to be simple to achieve logic applications without using three-terminal components (indeed, logic circuits based on Josephson junctions are difficult to set up).
2. Description of the Prior Art
A great many three-terminal applications have been proposed to date. These include, especially, a Josephson field-effect transistor (JOFET) as described in the document by A. W. KLEINWASSER et al. in "Superconducting Devices", S. Ruggiera and D. Rudman Academic Press Inc., San Diego, 1990. The working principle is identical to that of the FET except that the electrodes are made of superconducting material and that the semiconductor channel bears a superconductive current which is itself modulated by the gate voltage (see FIG. 1). The superconductive current may get propagated in the semiconductor by means of the well-known effect of proximity between normal metals and superconductors, which has also been demonstrated between semiconductors and superconductors.
However, no component of the JOFET group has yet been made with characteristics that are good enough to be useful.
Furthermore, the superconductivity of a layer (channel) may be controlled by modulating the electron density in certain particular systems. In lead salts, which are superconductors with a small forbidden band, small effective mass and high mobility, it is possible to prepare semiconductor layers having inclusions of metallic lead slugs as described in the document by S. Tanaka et al., "Controllability of Superconducting Behavior by Photo-Illuminations in Indium-Doped Pb.sub.1-x Sn.sub.x Te With Lead Inclusion", 20th Conference of Semiconductors, Thessaloniki, Greece, Aug. 6, 1990. At low temperatures, these inclusions become superconducting, and the existence of a superconducting current between these inclusions is a function of the electron density in the semiconductor matrix. This effect has been demonstrated in the document by Takaoka et al. Improvements in the critical temperature of the system have been obtained by making the electron density in the semiconducting matrix vary through the photoexcitation of electrons (illumination).
FIG. 2a shows, according to the Takaoka document, lead inclusions in a Pb.sub.1-x Sn.sub.x Te/In material and shows (in the hatched zones) the penetration of the superconducting state into the semiconducting material (Pb.sub.1-x Sn.sub.x Te/In).
FIG. 2b brings out the fact that, when this device is exposed to light, its superconducting regions get extended and its superconductivity increases.
The drawbacks of the approach using JOFETs are twofold. Firstly, to obtain superconductive currents that are not negligible when compared with the normal current, it is necessary to have channel lengths that are extremely small and hence difficult to manufacture. Secondly, the voltages between source and drain, at which the superconducting effects are great, are intrinsically limited by the gap energy of the Cooper pairs (about 1 mV in the older superconductors, and 10 mV in the new ones) while the gate voltages required to modulate the conduction in the channel are of the order of several hundreds of mV at the minimum.
The system described in the article by Takaoka et al. has the drawback of having very lengthy response times; the photo-excited electrons get recombined very slowly.